DataTable table = new DataTable();
table.Columns.Add("id", typeof(int));
table.Columns.Add("Field1", typeof(string));

table.Rows.Add(25, "01255055502555");
table.Rows.Add(50, "01255055522555");
table.Rows.Add(10, "01255055512555");
table.Rows.Add(21, "01255qwd522f55");
table.Rows.Add(100,"01255qwd511f55");

SELECT a1, Max(n) AS MaxOfn, Left([a1],Len([a1])-5) & Right([a1],4) AS field1
FROM (SELECT ID, Field1, Mid(Right([Field1],5),1,1) AS n, Left([field1],Len([field1])-6) & Right([field1],4) AS a1
FROM Table1)
GROUP BY a1;

    class BOMX_ResolutionTolls
    {
        private static String getCodeWithoutResolution(String s)
        {
            s = s.Replace(" ", "");
            s = s.Substring(0, s.Length - 5) + s.Substring(s.Length - 4, 4);
            return s;
        }

        private static String getValue_FiveFromEnd(String s)
        {
            s = s.Replace(" ", "");
            s = s.Substring(s.Length - 5, 1);

            return s;
        }

        private static String resolution(String s, string value)
        {
            return s.Insert(s.Length - 4, value);
        }

        private static DataTable getFieldWithResolutions(DataTable table, String fieldName)
        {
            DataTable t = new DataTable();
            t.Columns.Add("keyFld", typeof(string));
            t.Columns.Add("groupFld", typeof(string));
            t.Columns.Add("resulutionFld", typeof(string));

            var query = from b in table.AsEnumerable()
                        orderby b.Field<string>("Field1")
                        select new
                        {
                            keyFld = b.Field<String>("Field1"),
                            groupFld = getCodeWithoutResolution(b.Field<String>("Field1").ToString()),
                            resolutionFld = getValue_FiveFromEnd(b.Field<String>("Field1").ToString())
                        };

            foreach (var obj in query)
            {
                t.Rows.Add(obj.keyFld, obj.groupFld, obj.resolutionFld);
            }

            return t;
        }

        private static DataTable getGroup(DataTable table)
        {
            DataTable t1 = new DataTable();
            t1.Columns.Add("key", typeof(String));
            t1.Columns.Add("max", typeof(String));

            var query = from b in table.AsEnumerable()
                        group b by b.Field<string>("groupFld") into g
                        let list = g.ToList()
                        select new
                        {
                            key = g.Key,
                            max = list.Max(x => x.Field<String>("resulutionFld")),
                        };
            foreach (var obj in query)
            {
                t1.Rows.Add(obj.key, obj.max);
            }
            //------------------------------------------------------------
            DataTable t2 = new DataTable();
            t2.Columns.Add("key", typeof(String));
            t2.Columns.Add("max", typeof(String));
            t2.Columns.Add("text", typeof(String));

            var query3 = from b in t1.AsEnumerable()
                         orderby b.Field<string>("key")
                         select new
                         {
                             key = b.Field<String>("key"),
                             max = b.Field<String>("max"),
                             val = resolution(b.Field<String>("key"), b.Field<String>("max"))
                         };
            foreach (var obj in query3)
            {
                t2.Rows.Add(obj.key, obj.max, obj.val);
            }
            return t2;
        }

        public static DataTable getGroupResolutions(DataTable table)
        {
            DataTable query2 = new DataTable();
            DataTable query3 = new DataTable();

            query2 = getFieldWithResolutions(table, "Field1");
            query3 = getGroup(query2);

            return query3;
        }

    }